In recent years, because of ongoing improvements in the performance of liquid crystal display devices, further improvements in the brightness and higher levels of color reproducibility are being demanded for the backlight (the illumination device for the display device) used in transmission-type liquid crystal display panels. As a result, although combinations of fluorescent display tubes and light guide plates have conventionally been the predominant form of backlights, in recent years, backlights that employ LED devices have started to be used. Examples of these backlights that employ LED devices include devices in which a plurality of white LED devices are aligned on a substrate.
Examples of known white LED devices include white light-emitting diode devices composed of a combination of a blue light-emitting diode element (a blue LED chip) and a blue light-absorbing yellow light-emitting phosphor (namely, BY-type white LED devices), and white light-emitting diode devices composed of a combination of a blue LED chip, a blue light-absorbing green light-emitting phosphor, and a blue light-absorbing red light-emitting phosphor (namely, RGB-type white LED devices), and these devices are already in practical use in the backlights mentioned above.
As an example of the BY-type white LED device mentioned above, Patent Document 1 discloses a white light-emitting diode device that employs a combination of a blue light-emitting diode element and a blue light-absorbing yellow light-emitting phosphor. Further, Patent Document 2 discloses a light-emitting diode device of a similar configuration. Moreover, Patent Document 3 discloses a light-emitting diode device of a similar configuration as a light-emitting element that employs a wavelength-converting casting material.
As an example of the RGB-type white LED device mentioned above, Patent Document 4 discloses a phosphor-coated light-emitting diode which contains a semiconductor light-emitting element that emits ultraviolet light or near ultraviolet light, and a phosphor that is deposited on the surface of the element. In this structure, depending on the type of phosphor deposited on the surface of the element, the light emitted from this phosphor-coated light-emitting diode (LED device) may be blue, green or red. Further, Patent Document 5 discloses a dot matrix-type display device which contains a light-emitting layer composed of a group III nitride semiconductor, and three different phosphors, which receive the ultraviolet light having an emission peak wavelength of 380 nm emitted by the light-emitting layer, and emit light of the three primary colors of red, green and blue respectively.
The LED devices disclosed in Patent Documents 1 to 5 can be produced, for example, using conventional methods such as those disclosed in Patent Document 6 and Patent Document 7.
In these light-emitting diodes, one series of phosphors that is often used as a blue light-absorbing yellow light-emitting phosphor is the cerium-activated yttrium-aluminum-garnet (YAG)-based oxide phosphors represented by the general formula: (Y,Gd)3(Al,Ga)5O12:Ce3+. 
The above-mentioned BY-type white LED devices have tended to suffer from a problem wherein the red component is inadequate, leading to a bluish white emission that results in an observed bias in the color rendering properties. Further, as the brightness of the above blue LED chip is increased, the amount of heat generated also increases, and some phosphors have suffered from a problem wherein the heat causes a portion of the phosphor to decompose and stop emitting light, leading to a reduction in the light emission brightness of the LED chip. Moreover, in the case of YAG phosphors and the like, because they suffer from reduced conversion efficiency at high temperature, a problem wherein the light emission intensity decreases rapidly under high-temperature environments has also sometimes occurred.
Besides the yttrium-aluminum-garnet (YAG)-based oxide phosphors mentioned above, other known examples of blue light-absorbing yellow light-emitting phosphors are sulfide-based phosphors. For example, Patent Document 8 discloses a white light-emitting semiconductor light-emitting element which uses a semiconductor light-emitting element that emits light of a wavelength of 390 to 420 nm, and a phosphor that is excited by the light emitted from this semiconductor light-emitting element. All manner of oxide and sulfide phosphors can be used as the phosphor that is excited and emits light upon irradiation with the light of wavelength of 390 to 420 nm. However, in the case of the sulfide-based phosphors, chemical stability has been a problem, and the necessary lifespan required for use as a white light LED device has not always been achievable.
Examples of known blue light-absorbing yellow light-emitting phosphors besides the phosphors mentioned above include silicate phosphors, phosphate phosphors and aluminate phosphors. However, these phosphors also suffer from a decrease in the light emission brightness of the phosphor upon exposure to excitation sources having a high level of energy such as vacuum ultraviolet light, ultraviolet light, electron beams and blue light.
On the other hand, oxynitride phosphors such as sialon phosphors are reported to undergo minimal deterioration in the brightness even when exposed to the above-mentioned excitation sources. For example, Patent Document 9 discloses a sialon phosphor containing Ca. This sialon phosphor is produced by first mixing silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3) and europium oxide (Eu2O3) in a predetermined molar ratio, and then performing a calcination using a hot press method by holding the mixture for one hour within 1 atmosphere of nitrogen (0.1 MPa) at a temperature of 1,700° C. The α-sialon phosphor containing a solid solution of Eu ions obtained using the above method is a blue light-absorbing yellow light-emitting phosphor that is excited by blue light of 450 to 500 nm and emits yellow light of 550 to 600 nm.
Further, Patent Document 10 relates to a different sialon phosphor, and discloses a β-sialon phosphor having a β-Si3N4 structure. This β-sialon phosphor is a blue light-absorbing yellow light-emitting phosphor that emits green to orange light of 500 to 600 nm upon excitation with ultraviolet light to blue light.
Patent Document 11 discloses an oxynitride phosphor composed of a JEM phase. This oxynitride phosphor is a blue light-absorbing green light-emitting phosphor that is excited by ultraviolet light to blue light, and emits light having an emission wavelength peak of 460 to 510 nm.